I also opted for notification if someone defriends me. Sometimes I get the notification, sometimes I don't. On my PC I used to use a fairly nifty LJ client called Semagic, and it always tells you when stuff like that happens. Sadly Semagic doesn't work on a Mac, and there don't seem to be any decent LJ clients for the Mac.

After five years on LJ I'm still not entirely certain of the whole friending/defriending etiquette.